It really may depend on why you are placing a value on your model collection. Is it to place a value on your personal property so you can set the limit of coverage you are buying or are you purchasing coverage specific to the collection? If you are doing this to place a value on your personal property, you may want to value the partial kits at full retail value. You are over valuing them, but you do not want to under insure your things. If you are buying coverage specifically for the collection, you should be very realistic with what you have and value the partial kits accordingly. If you have a claim in this instance the last thing you want to have happen is a claims adjuster look in a box and see that much of what is supposed to be there is missing. Depending on what is damaged, they may not care that the engine is in a particular competed model or other parts are on different models. And proving conclusively that the parts that are not in the box, but are on completed models may be very difficult. Many states have fraud statutes and you do not want to get cross ways here as it can jeopardize the entire claim. If this is wonky, I apologize, but in a prior career, I used to be an insurance regulator. Insurance companies can be real prickly at times and you do not want to give them reason to question your claim.